RPS attempts to distribute incoming packet load amongst multiple CPUs, but does not take into account any data locality issues for maximizing CPU cache hit rates. You can use RFS to help increase cache hit rates by directing packets for the same flow to the same CPU for processing. Thus, netif_receive_skb will either continue sending network data up the networking stack, or hand it over to RPS for processing on a different CPU. The function napi_gro_receive deals processing network data for GRO and sending the data up the stack toward the protocol layers. Much of this logic is handled in a function called dev_gro_receive.

This makes sense as this small stub of code is only executed when more work could have been done, but we don’t want to monopolize the CPU. One important piece of information about the contract between the NAPI subsystem and device drivers which has not been mentioned yet are the requirements around shutting down NAPI. If you don’t have enough CPUs to distribute your NIC’s IRQs, you can consider increasing the net_rx_action budget to allow for more packet processing for each CPU. Increasing the budget will increase CPU usage , but should reduce latency as data will be processed more promptly.

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RFS can be sped up with the use of hardware acceleration; the NIC and the kernel can work together to determine which flows should be processed on which CPUs. To use this feature, it must be supported by the NIC and your driver.

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One interesting option that some drivers support is “adaptive RX/TX IRQ coalescing.” This option is typically implemented in hardware. The driver usually needs to do some work to inform the NIC that this feature is enabled and some bookkeeping as well . Interrupt coalescing is a method of preventing interrupts from being raised by a device to a CPU until a specific amount of work or number of events are pending.

If you are not using RPS and your driver is not using netif_rx, increasing the backlog won’t produce any noticeable effect on your system as it is not used. netif_receive_skb begins by first checking a sysctl value to determine if the user has requested receive timestamping before or after a packet hits the backlog queue. If this setting is enabled, the data is timestamped now, prior to it hitting RPS (and the CPU’s associated backlog queue). If this setting is disabled, it will be timestamped after it hits the queue. This can be used to distribute the load of timestamping amongst multiple CPUs if RPS is enabled, but will introduce some delay as a result.

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Otherwise, the driver turns off NAPI by calling napi_complete and re-enables interrupts by calling igb_ring_irq_enable. The struct softnet_data structure has some statistics incremented and the softirq NET_RX_SOFTIRQ is shut down. The time_squeeze field is a measure of the number of times net_rx_action had more work to do but either the budget was exhausted or the time limit was reached before it could be completed. This is a tremendously useful counter for understanding bottlenecks in network processing. The NET_RX_SOFTIRQ is disabled to free up processing time for other tasks.

net_rx_action begins the processing of packets from the memory the packets were DMA’d into by the device. If your NIC supports RSS / multiqueue or if you are attempting to optimize for data locality, you may wish to use a specific set of CPUs for handling interrupts generated by your NIC. Reminder that your hardware and driver may only support a subset of the options listed above. You should consult your driver source code and your hardware data sheet for more information on supported coalescing options.

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